Manually Set Asterisk Network IP Address -- Contact and Network IP Mismatch

On some networks, ours included, multiple external IP addresses can be assigned to a single port.  For example, you purchase a block of five IP addresses from your ISP.  All five IP's are then assigned to the WAN port of the firewall/router.

FreePBX automatically queries the firewall and gets a response under CONNECTIVITY->SIP STATION.  Clicking the Check Connectivity button returns the first IP address assigned to the WAN port. 

The Issue:  We need to assign one of the other IP addresses listed on the WAN port.  Another FreePBX server has already grabbed the first listed IP address and is in use on the "OLD" FreePBX server.  Installing a "NEW" FreePBX server requires one of the other static IP addresses.

The Problem:  Have not been able to assign one of the other available static IP addresses.  We have attempted to locate the Asterisk file that controls this but did not find the appropriate file. 

Attempted Fix:  Have reviewed  Asterisk Config Files: pjsip.transports.conf and sip_general_additional.conf.  The files reflect the correct "Your Contact IP" but no mention of the "Your Network IP".  I suspect that there is a specific FreePBX file for this configuration but have not located it.  Perhaps this is a feature request?

Request:  How can I manually set the "Your Network IP" so that it matches the "Your Contact IP"?  How do we manually assign IP addresses?

Additional Info:  I have manually set the IP address in Asterisk SIP Settings on both the General SIP Settings and the SIP Setting [chan_pjsip] tabs.  The correct (desired) IP address was assigned to External Address and External IP Address boxes.

Note:  I've added redacted screenshots for guidance.